a. Field of the Invention
The present invention relates to an optical scanning system and more particularly relates to an optical scanning system including an optical system for correcting the error caused by inclination of a deflecting device from its normal position.
b. Description of the Prior Art
In a scanning system using deflecting mirror surfaces rotating about an axis such as a polygon mirror or galvano mirror, there may occur such unfavourable phenomenon that the deflecting mirror surface tilts in the direction normal to the deflection surface in such manner as to produce an error of the position of the deflection. Such error is known as tilt error. If such an error in position occurs, then the deflection scanning will be deviated in the direction normal to the deflection surface so that the uniformity in pitch of the scanning lines may be disturbed. Therefore, this phenomenon should be corrected and an optical system used for this purpose of correction is called a tilt error correcting optical system. Herein, the term "deflection surface" means a surface which a normal line to the deflecting mirror surface describes with time as the deflecting mirror rotates.
Various optical scanning systems provided with a tilt error correcting optical system have been proposed and are known in the art. In view of the optical relation between the deflecting mirror surface and the scanned surface, these known optical scanning systems are divided into two groups, the conjugate type and the non-conjugate type.
It is also an important problem for the optical scanning optical system to keep constant the moving speed of the deflected beam spot on the scanned surface. Generally, the scanning speed of the beam spot is determined by the characteristics of the rotational movement of the deflecting device used as well as by the distortion characteristics of the lens used for scanning.
The above mentioned conjugate type of optical scanning system is featured by the fact that the deflecting mirror surface and the scanned surface are in a conjugated relation relative to the optical system interposed therebetween in a plane normal to the above defined deflection surface to correct any tilt error. Typical examples of this type of optical scanning system are disclosed in U.S. Pat. Nos. 3,750,189, 3,865,465 and 3,946,150.
However, in U.S. Pat. Nos. 3,750,189 and 3,865,465 there is no description of a technique for moving the beam spot at uniform speed on the scanned surface. In U.S. Pat. No. 3,946,150, there is a description of a technique for moving the beam spot at uniform speed. According to the teaching, uniform speed of scanning beam spot and correction of tilt error are attained by using an anamorphic lens system and a spherical scanning lens. However, such a combination of an anamorphic optical system and a spherical optical system is very difficult to design because the combination must be disposed between the deflecting device and the scanned surface so as to satisfy both of the uniformity of beam spot moving speed and the necessary correction of tilt error.
The non-conjugate type of optical scanning system is featured by the fact that the deflecting mirror surface and the scanned surface are in a non-conjugated relation with respect to the optical system disposed therebetween in a plane normal to the deflection surface. The optical system is so designed, to correct tilt error, that its focal length measured along the deflection surface becomes longer than that measured along a plane normal to the deflection surface. Typical examples of this type of optical scanning system are disclosed in U.S. Pat. No. 3,877,777, DAS. No. 2,517,821 and U.S. Pat. No. 4,056,307. Among these publications, U.S. Pat. No. 3,877,777 contains no teaching of means for attaining the uniformity of beam spot moving speed on the scanned surface. DAS. No. 2,517,821 contains some teaching about it according to which an anamorphic optical system and a spherical scanning lens are disposed between the deflecting device and the scanned surface. The last mentioned U.S. Pat. No. 4,056,307 proposed to dispose only an anamorphic optical system between the deflecting device and the scanned surface in order to satisfy the requirements of tilt error correction and uniform speed of moving beam spot. However, this arrangement requires six cylindrical lenses to attain the necessary uniformity of beam spot moving speed and therefore the lens system for scanning takes an extremely large space. No compact structure is obtainable according to this prior art.